FIELD OF THE INVENTION
The invention relates to a method and device for contact-free retention of sheets on the circumferential surface of a cylinder or a planar guiding surface of a sheet-processing machine, in particular, a sheet-fed rotary printing machine.
The published German Patent Document DE 198 15 794 A1 has already disclosed heretofore a practice of installing, on a transfer cylinder of a sheet-processing machine, blowing/suction elements which subject to suction action a sheet transported primarily by grippers, and thus retain the sheet on the circumferential surface of the cylinder. The blowing or blast air that is applied here flows out parallel to and beneath the sheet and, assisted by suitably provided suction openings, against the cylinder body. In the case of sheets which are freshly printed on the reverse side thereof, contact between the sheet and the cylinder body may result in smearing problems.
It is accordingly an object of the invention to provide a method and a device for implementing the method, by which, on a sheet-transporting cylinder or a guiding surface in a sheet-processing machine, the sheet is subjected to retaining forces which retain the sheet in a more-or-less contact-free manner in the vicinity of the circumferential surface of the cylinder or of the sheet-guiding surface.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a method of retaining a sheet on at least one of a transfer cylinder and a guide surface of a sheet-guiding device in a sheet-processing machine by pneumatically activatable nozzles for producing an air cushion beneath the respective sheet, which comprises producing an air cushion by an air-pressure distribution resulting in air flows beneath the respective sheet, the air-pressure distribution being based virtually only on radially decelerated air flow, and being capable of retaining the respective sheet in position.
In accordance with another mode, the method of the invention includes producing an air pressure adapted to the weight per unit area of the sheet.
In accordance with a further mode, the method of the invention includes setting the air pressure at between 5,000 and 500,000 pascals.
In accordance with another aspect of the invention, there is provided a device for retaining sheets on at least one of a transfer cylinder and a guide surface of a sheet-guiding device in a sheet-processing machine, comprising pneumatically activatable nozzles for producing an air cushion beneath the respective sheets, the nozzles having respective central blast-air openings as a center thereof, and an outflow feature selected from the group consisting of outflow openings arranged in a circle around the center and at least one outflow channel.
In accordance with a further aspect of the invention, there is provided a device for retaining sheets on at least one of a transfer cylinder and a guide surface of a sheet-guiding device in a sheet-processing machine, comprising pneumatically activatable nozzles for producing an air cushion beneath the respective sheets, the nozzles constituting a swivel nozzle as a center, and an outflow feature selected from the group consisting of outflow openings arranged in a circle around the center and at least one outflow channel.
In accordance with another feature of the invention, the blast-air opening is a borehole, and the outflow openings are boreholes.
In accordance with a further feature of the invention, the swivel nozzle has at least two boreholes which are offset relative to one another and are inclined relative to an outlet surface.
In accordance with an added feature of the invention, an angle smaller than 60xc2x0 is enclosed by the boreholes, respectively, and the outlet surface.
In accordance with an additional feature of the invention, the boreholes have an angular offset from one another at least approximately resulting from a division corresponding to the number of boreholes.
In accordance with yet another feature of the invention, the blast-air opening is connected to a pressure source, and the outflow openings are connected to atmosphere.
In accordance with yet a further feature of the invention, the swivel nozzle is connected to a pressure source, and the outflow openings are connected to atmosphere.
In accordance with yet an added feature of the invention, the blast-air opening is a ring-gap nozzle.
In accordance with yet an additional feature of the invention, the ring-gap nozzle is formed of a number of boreholes arranged in a circle.
In accordance with still another feature of the invention, the blast-air openings are able to be supplied with a pressure of from 5,000 to 500,000 pascals.
In accordance with still a further feature of the invention, the swivel nozzles are able to be supplied with a pressure of from 5,000 to 500,000 pascals.
In accordance with a concomitant feature of the invention, the sheet-processing machine is a rotary printing machine.
A particular advantage of the invention is that of retaining the sheet on an air cushion produced by a free blast-air jet which produces an air flow beneath the sheet and, utilizing the hydrodynamic paradox, subjects the sheet to a retaining force. In particular, when used on transfer cylinders wherein the freshly printed side of the sheet is directed towards the circumferential surface of the cylinder, the method and device according to the invention avoid smearing of the fresh ink on the cylinder. When the invention is applied to a storage drum, the sheet can be clamped easily by transfer systems acting upon the trailing edge thereof.
A further location for a planar sheet-guiding surface used with the invention is, for example, a feed table, in which case the sheet is aligned initially at the leading edge thereof and then laterally. So-called front blowers, which blow on the leading edge of the sheet counter to the sheet-transporting direction, aid the separation from a following sheet of the sheet which is to be aligned. In order that the sheet which is to be aligned is not blown upwardly from the feed table, however, provision is made for applying the method and device according to the invention in order, on the one hand, to retain the sheet on the feed table and, on the other hand, to facilitate the lateral alignment thereof on an air cushion.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for a contact-free retention of sheets, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: